A Circular Polarization Selective Surface (CPSS) is a finite-thickness surface that predominately reflects one sense, or handedness, of a circular polarization (CP) of an incident electro-magnetic (EM) wave, and predominantly transmits the other sense of CP. An ideal reciprocal CPSS acts either as a mirror or a transparent window, depending on the sense of CP of the incident wave. A reciprocal CPSS is one for which the sense of CP of the predominantly reflected wave is the same as that of the incident wave. This is opposite to an ordinary reflection from an interface between two dielectric media or from a common metallic mirror, wherein the sense of the predominant CP of the reflected wave is opposite to that of the incident wave. Furthermore, the general operation of a reciprocal CPSS typically remains the same regardless of whether the CPSS is illuminated from one side or the other. In its simplest form, a prior art CPSS is a two-Dimensional (2D) periodic array of identical CPSS elements that lacks longitudinal reflection symmetry, is reciprocal, and with a Cartesian tiling configuration. In the context of this specification, the longitudinal direction is the direction that is normal to the CPSS. A CPSS is typically designed to CP-selectively reflect or transmit incident EM radiation of a particular frequency f, which is referred to hereinafter as the operating frequency, or simply the frequency. The wavelength λ corresponding to the frequency f depends on the effective permittivity of the propagation medium.
U.S. Pat. No. 3,500,420 issued to Pierrot discloses an example of a CPSS array, wherein the CPSS element is a single crankwire that is illustrated in FIG. 1. Here, a crankwire is a conductive wire that is bent to be comprised of three mutually perpendicular conducting segments. In the Pierrot design, the lengths of two perpendicular end segments, which are also referred to herein as transverse segments (TS), is 3λ/8, while the length of the middle, or longitudinal, segment is λ/4, with the total length of the crankwire equal to one wavelength λ. The relative orientation of the two transverse segments, i.e. the handedness of the geometry, dictates the operation of the CPSS element as to which sense of CP will be reflected upon being illuminated with a CP plane wave incident in the normal direction, i.e. a direction parallel with the longitudinal segment. Using Cartesian notation, when the longitudinal segment is aligned with the Z direction as illustrated in FIG. 1, the bottom transverse segment is aligned with the +X direction and the top transverse segment is aligned with the +Y direction, the crankwire reflects Left-Hand Circular Polarization (LHCP) when illuminated from the top or bottom, and a corresponding CPSS is referred to as a LHCPSS. With the top transverse segment aligned with the +X direction and the bottom transverse segment aligned with the +Y direction, the crankwire reflects Right-Hand Circular Polarization (RHCP) when illuminated from the top or bottom, and a corresponding CPSS is referred to as a RHCPSS. The crankwire has the same general operation whether it is illuminated from one end of its longitudinal axis or the other.
The two in-phase currents cooperate to produce a strong scattering response whereas the two out-of-phase currents nearly cancel one another to produce a weak scattering response. With the in-phase condition, the one-wavelength crankwire becomes resonant so that the current distribution over the entire length of the wire is sinusoidal-like, with a peak on each transverse segment and a null at the mid-point of the longitudinal segment. The relative orientation of the transverse segments that determines the handedness of the crankwire, and the λ/4 spacing between the transverse segments ensure that the sense of CP of the reflected wave is the same as that of the incident wave. Hence, the reflected wave is strong and the sense of its CP is the same as that of the incident wave. In contrast, the total transmitted field is very weak because the transmitted scattered wave is equal and opposite to the incident wave, and because the total transmitted field is the vectorial summation of the incident wave and the scattered wave. With the out-of-phase condition, the two out-of-phase currents produce a bell shape current distribution with a small peak value at the mid-length point of the longitudinal segment. Since this produces only a very weak scattering response, the incident wave goes through the crankwire with little or no disturbance as if the crankwire were absent.
A variation of the Pierrot design using printed circuit boards with metalized via-holes to implement the crankwires is disclosed in an article by I-Young Tarn and Shyh-Jong Chung, “A New Advance in Circular Polarization Selective Surface—A Three Layered CPSS Without Vertical Conductive Segments”, IEEE Transactions on Antennas and Propagation, Vol. 55, No. 2, February 2007, pp. 460-467, which is incorporated herein by reference. It involves using the Printed Circuit Board (PCB) technology to implement the crankwires, with the metallized via-holes that realizes the longitudinal segments of the crankwires being replaced by conducting traces on intermediate layers between the top and bottom surfaces of the PCB. Due to the partial vertical alignment of one strip with the strip on the next layer, the EM energy flows vertically from one strip to the other by capacitive coupling. This permits to electrically connect the two transverse segments of the crankwire without using a continuous conductor between them. The insertion loss resulting from this arrangement may be, however, large (e.g. about 2.3 dB).
A drawback of CPSS of the Pierrot type composed of a periodic array of the crankwires of the same handedness is that its performance quickly degrades with oblique incidence.
U.S. Pat. No. 5,053,785 to Tilston et al., which is incorporated herein by reference, discloses a CPSS element 20 in the form of a dipole arrangement that is illustrated in FIG. 2, and which has a 2-fold rotational symmetry. The CPSS element 20 of Tilston includes two perpendicular half-wavelength dipoles 22 and 24 separated physically by a λ/4 spacing but connected electrically by a λ/2 transmission line 30. One advantage of the Tilston's design is that is has a 2-fold rotational symmetry, which symmetry has been shown in Jasmin E. Roy, “Reciprocal Circular Polarization Selective Surfaces”, Ph.D. thesis, University of Manitoba, Winnipeg, Manitoba, December 1995 to provide a good performance under oblique incidence.
Notably, U.S. Pat. No. 5,053,785 is silent as to possible solutions to a problem of incorporating the half-wavelength transmission line in the quarter-wavelength spacing that corresponds to the thickness of the cell, and further is silent on possible performance of the suggested design. Furthermore, the half-wavelength dipoles need to be rotated 45 degrees to lie on the diagonals of the cells in order to fit within cells that are no larger than a half-wavelength in order to avoid the formation of grating lobes and the presence of higher-order modes of propagation.
FIG. 3 illustrates another CPSS that may be referred to as a CP-LP-CP cascade design, which is disclosed by U.S. Pat. No. 3,271,771 to P. W. Hannan et al. It includes a cascade of two circular polarizers of opposite handedness sandwiching a linear wire-grid polarizer. Its operation involves converting the input CP into a Linear Polarization (LP), filtering the LP with a wire-grid and reconverting the output LP into CP. The CPSS operation would be changed from reflecting one sense of CP to reflecting the other sense of CP by rotating the wire-grid by 90 degrees. One disadvantage of the cascade design is that its performance under oblique incidence is limited because the linear polarization filter works best only under normal incident EM illumination. Also, the realization of the CP-LP-CP cascade design is much thicker than those of Pierrot's or Tilston's designs, which is a disadvantage in terms of volume, weight and space.
An object of the present disclosure is to provide an improved CPSS which addresses at least some of the disadvantages of the prior art, and which provides improved performance in at least some applications.